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Tuesday, December 08, 2009

The CREATE ANY TRIGGER three card trick.

I've got an interest in security, or at least in vulnerabilities. It isn't something I've really had as part of a job role but I like the tricky way of thinking it entails, and I follow a number of security blogs (both Oracle and generic).

During my degree course, when I was mooching around the Prime computers the students had accounts on I discovered some forgotten directories which allowed me to stop worrying about the number and size of files I wanted to store. My degree course was a 'sandwich' one, where the third year was spent working in the real world, and while there I discovered another 'hole'. In that case it was on a *nix system where, although the login account went straight to a menu rather than giving shell access, I could execute shell commands from within the text editor.

After graduating, I was working for a software provider. One client had an important batch job which failed to run at a weekend. Some clients had support accounts that we could access anytime, others required the account to be unlocked, and one site had to turn on the modem before we could connect. This site was quite secure and I couldn't get to the support account but it was local enough that I could drive to the site. The guy I was dealing with had a basic login with just a list of menu options so I still didn't have any command line or SQL level access to fix the problem. Remembering my previous adventures, I nipped into the MAIL option and from there to the text editor. [Hey, this was pre-internet and pretty much pre-Windows. You sent and read your mail through green-screen terminals.] This was VMS and the environment was smarter than the *nix environment had been and knew I was limited to a locked down account, so the editor wouldn't allow shell commands. But it would let me open up another file in the editor, and so I could edit a file that would be run by the batch job when it restarted. See what I mean about twisty thinking ?

DBAs need to understand the twisty thinking when it comes to protecting databases. So I though I'd push out an Oracle example of twisty thinking.

DBAs know that the *ANY* privileges are generally dangerous and shouldn't be handed out unless absolutely necessary (and even then, with lots of auditing and a large spiky stick). CREATE ANY TRIGGER is one of the trickier ones. On the face of it, it allows a user to create a trigger on anyone's table (or a DDL or event trigger). The trick is that, table triggers do NOT need to be owned by the same schema as that of the table.

So a user, DICK, with 'CREATE ANY TRIGGER' can create a trigger owned by TOM on a table owned by HARRY. That way anyone who inserts, updates or deletes a row in HARRY's table will fire the trigger that runs with TOM's privileges. And given DICK can create the trigger with any owner he wants, the TOM account will be the one with the most privileges.

Obviously, DBAs don't go around granting CREATE ANY TRIGGER to just anyone. A measly developer may just get an account with CREATE TABLE, but that's about it. Once you consider that the developer's account can be the table owner though (HARRY in the example), we have our first twist. A sneaky cracker will need a powerful DICK to create the trigger so he looks around for an account that already has that privilege, such as MDSYS (which owns the spatial datatypes, packages etc). That account is probably LOCKED and EXPIRED, but that doesn't make it unusable. A cracker will be studying the Oracle CPUs and security sites for SQL or PL/SQL injection vulnerabilities in those privileged (and locked down) accounts and those will be used to get access to the CREATE ANY TRIGGER privilege.

To summarise, cracker gets a lowly priviliged HARRY account, uses vulnerabilities in packages owned by default DICK users (despite them being locked) to create triggers owned by TOM users with very high privileges on HARRY's own tables. By inserting into those tables, the cracker can fire a trigger to grant himself DBA or whatever.

From there you can use CREATE ANY DIRECTORY to start hunting around the server file system, and write ssh or rhosts files to get to the oracle login. That may only be on a development system, but some snazzy rootkit style executables there could help the cracker make the jump to production. Or maybe, by pointing the CREATE ANY DIRECTORY cannon at the directory with the Oracle data files, they use UTL_FILE to read the files/blocks relating to C_USER# (the data segment underlying dba_users) and get the password hashes (which are no longer visible in DBA_USERS in 11g). There are some fun things being done to speed up cracking of Oracle passwords so anyone thinking the eight-character alphanumeric password they have been using for a couple of months is safe should think again. You don't use the same password for anything important do you ?

So, have a think about your password complexity and change requirements, applying those CPUs to those development and test databases as well as production, and actively auditing those more powerful privileges (by which I mean you READ about any use of them when they happen, and not six months later).